1. Field of the Invention
The disclosures herein generally relate to a magnetic recording medium and a magnetic recording and reproducing apparatus.
2. Description of the Related Art
Currently, a hard disk drive (HDD) that is a type of magnetic recording and reproducing apparatus has been exponentially increasing its recording density, and the recording density is said to be likely to continue to increase in the future. Therefore, a magnetic head and a magnetic recording medium suitable for recording density growth have been developed.
A magnetic recording medium, mounted on the magnetic recording and reproducing apparatus commercially available at present, is a so-called perpendicular (vertical) magnetic recording medium in which an easy axis of magnetization within a magnetic film is oriented mainly perpendicularly. The perpendicular magnetic recording medium can prevent noise from increasing because the perpendicular magnetic recording medium is less subject to a diamagnetic field in a boundary region between recording bits when increasing the recording density and a clear bit boundary is formed. Moreover, because a decrease in recording bit volume along with the recording density growth can be lessened, the perpendicular magnetic recording medium is resistant to a thermal fluctuation effect. In light of this, there is proposed a structure of a medium suitable for the perpendicular magnetic recording, this structure attracting remarkable attention in recent years.
In order to increase the recording density of the magnetic recording medium, it is necessary to facilitate magnetic separation of crystal particles that constitute a magnetic recording layer and to reduce a unit of magnetization reversal. However, this leads to a decrease of thermal stability of the magnetic recording medium. Therefore, in order to maintain the thermal stability of the magnetic recording medium, it is necessary to increase a magnetic anisotropic constant (Ku) of a magnetic material that constitutes the magnetic particle.
In order to constitute such a magnetic recording medium, it is preferable that the magnetic recording layer has (adopts) a granular structure using the magnetic material having high Ku. However, the magnetic recording medium, using the magnetic material having high Ku, has problem points that strength of a magnetic field required in recording increases and a write-ability (easiness of recording) of the magnetic recording medium decreases.
In order to solve the problem points, a configuration, in which a plurality of magnetic recording layers are layered via exchange coupling control layers to ferromagnetically couple the magnetic particles that constitute each layer, is known in the related art. In the plurality of magnetic recording layers of the configuration, magnetization of magnetic particles having low Ku is reversed previously, and magnetization of magnetic particles having higher Ku is reversed. That is, in a case where the plurality of magnetic recording layers are layered without the exchange coupling control layers, the magnetization of these magnetic recording layers is simultaneously reversed and the write-ability of the magnetic recording layers decreases. On the other hand, the write-ability of the magnetic recording medium can be enhanced by using the above described structure (for example, see Patent Documents 1 to 3).
Also, Non-Patent Document 1 discloses an anisotropy field Hk and a saturation magnetization Ms of a CoCrPt alloy film.
There are no bounds to the request for high recording densification of a magnetic recording medium, and the magnetic recording medium is required to have highly improved thermal fluctuation characteristics and highly improved write-ability more than ever. In order to fulfill such a request, in the magnetic recording medium, the number of magnetic recording layers increases. In accordance with the increase of the number of magnetic recording layers, the number of exchange coupling control layers, which control ferromagnetic coupling between the respective magnetic recording layers, increases.
According to examination by the present inventors, it is found that ATI/FTI (ATI: Adjacent Track Interference, FTI: Far Track Interference) tolerability of the magnetic recording medium degrades when the number of exchange coupling control layers increases in the laminated structure of the magnetic recording medium. It is considered that this is because, when the number of exchange coupling layers increases, a magnetization reversal area of the lower layer side of the magnetic recording layers expands particularly, and a bit boundary is disordered. As a result, the ATI/FTI tolerability of the magnetic recording medium degrades. Further, it is considered that because a main magnetic volume of the magnetic head realizing the high recording densification is small, a recording magnetic field leaks from other positions of the magnetic pole, and the leaked magnetic field deteriorates information of a recording track adjacent to the magnetic pole position where the magnetization reversal easily occurs in a weak magnetic field.